1. Field of the Invention
The invention relates to methods and systems for vehicle exhaust gas treatment to provide reduced vehicle tailpipe emissions when the vehicle""s engine is operated lean of stoichiometry.
2. Background Art
The operation of a typical hydrocarbon-fuel, fuel-injected internal combustion engine, as may be found in motor vehicles, results in the generation of engine exhaust gas which includes a variety of constituents including, for example, nitrogen oxides (NOx). The rates at which an engine generates such constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR.
In order to comply with modern restrictions regarding permissible levels of tailpipe emissions, the prior art teaches placement of an emission control device in the vehicle exhaust system. The device operates to xe2x80x9cstorexe2x80x9d one or more selected exhaust gas constituents when the exhaust gas is xe2x80x9cleanxe2x80x9d of stoichiometry, i.e., when ratio of intake air to injected fuel is greater than the stoichiometric air-fuel ratio. The device further operates to xe2x80x9creleasexe2x80x9d, at least some of the previously-stored exhaust gas constituents when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio.
Because continued lean operation of the engine will ultimately xe2x80x9cfill upxe2x80x9d or saturate the device with the selected exhaust gas constituents, the prior art teaches periodically varying the air-fuel ratio between a nominally lean setting to a rich setting, during which stored constituent gas is released from the device and reduced by the available hydrocarbons in the enriched operating condition. The period during which the exhaust gas constituents are stored in the device is generally referred to as xe2x80x9cfill time,xe2x80x9d while the period during which NOx is released or xe2x80x9cpurgedxe2x80x9d from the device is generally referred to as xe2x80x9cpurge time.xe2x80x9d The device fill and purge times must be controlled so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing the output of the constituent gas in the vehicle exhaust emissions.
It is an object of the invention to provide a method and system for controlling the exhaust gas emissions of an internal combustion engine which capitalizes upon the presence of an exhaust emission control device to enhance emissions reduction when operating the engine using near stoichiometric air-fuel mixtures.
Under the invention, an emission control device is preconditioned so that the device can operate to efficiently remove exhaust gas constituents, such as HC, CO and NOx, during a subsequent closed-loop, near-stoichiometric engine operation, and to better tolerate brief lean or rich transients. The device is preconditioned by oxidizing/reducing only the oxygen and the exhaust gas constituent(s) which have been previously stored in an upstream portion of the device during a lean engine operating condition, thereby leaving some oxygen and exhaust gas constituents stored in the device""s downstream portion. The oxygen and exhaust gas constituents remaining in the downstream portion of the preconditioned device permit the device to act like a typical catalytic emission control device to thereby provide higher emissions reduction during closed-loop operation near stoichiometry. Thus, for example, if some oxygen and NOx remain within the downstream portion of the preconditioned device, any excess HC and/or CO produced during the subsequent near-stoichiometric engine operating condition will be accommodated by the remaining oxygen and NOx in the downstream portion of the preconditioned device. Similarly, excess NOx generated in the subsequent near-stoichiometric engine operating condition will be stored in the upstream portion of the device.
In accordance with the invention, when the device is to be preconditioned for subsequent engine operation about stoichiometry, the engine is operated at a first engine operating condition characterized by combustion of a first air-fuel mixture having a first air-fuel ratio lean of the stoichiometric air-fuel ratio is employed to substantially xe2x80x9cfillxe2x80x9d or saturate the device""s media with oxygen and the selected exhaust gas constituent(s), such as NOx. The engine is then operated at a second operating condition characterized by combustion of a second air-fuel mixture having a second air-fuel ratio rich of the stoichiometric air-fuel ratio. The second operating condition is continued until excess hydrocarbons have xe2x80x9cbroken throughxe2x80x9d the upstream portion of the device""s media, whereupon the engine is operated at a third operating condition characterized by closed-loop variation of the air-fuel mixture supplied to the engine about the stoichiometric air-fuel mixture.
In accordance with another feature of the invention, in a preferred embodiment, a sensor, positioned within the device between the upstream portion and the downstream portion, generates an output signal representing the concentration of oxygen in the exhaust gas flowing through the device at a position within the device between the upstream portion and the downstream portion. The sensor output signal is used to determine the time period during which the engine is operated at the second operating condition, for example, by comparing the sensor output signal with a reference value. The second time period is further preferably adjusted to reflect the amount of fuel, in excess of the stoichiometric amount when operating at the second air-fuel ratio, which is already in the exhaust system, between the engine and the mid-device sensor, at the time that the sensor""s output signal indicates the presence of excess hydrocarbons in the exhaust gas flowing through the device.
Most preferably, and in accordance with another feature of the invention, the oxygen sensor is itself positioned longitudinally within the device to accommodate the excess fuel that is already in the exhaust system, upstream of the device, when the sensor""s output signal indicates the presence of excess hydrocarbons in the device, thereby simplifying the control process for preconditioning the device.
The above object and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.